There is a potential problem with this plan, apart from the question of whether Trump can do this by executive order rather than requiring an Act of Congress.

The problem is the Anti-Commandeering doctrine, which states that the federal government cannot dictate to local police forces what they must do. That "commandeers" state powers into the service of the federal government, which runs afoul of the Constitution, the 10th Amendment, specifically.

You might say, "But they're not forcing them to do this, they're just withholding money if they don't." True. But, if you remember the Supreme Court arguments over Obamacare, the conservative justices argued persuasively that to take away all of a state's federal medicaid subsidies if they did not expand Medicaid per the federal law's dictates would run afoul of the anti-commandeering doctrine.

The Supreme Court voided that part of Obamacare as unconstitutional.

Now, I do think you're permitted to withhold a fair amount of money-- the national government withheld highway funds from states that didn't go along with their 21-years-old-to-drink initiative, if I remember right. And they probably did that to make the 55 mph speed limit national (for the period when it was in fact national).

So I think you can construct a withhold-some-money scheme that passes muster. But you can't go too far with it, or the Supreme Court will strike it down.

And then there's that first problem of doing by executive order what Congress has not approved.

As reducing aid to cities can be construed as a budgetary concern (it reduces federal outlays, after all), I suppose this could be passed via reconcilliation (thus avoiding any Senate filibuster), but you'd still need Congress to go along, and you'd still need 50 of our 52 Senators to go along, and a lot of them are RINOs.

Might be worth trying anyway, just to flush them out and expose them to primary challenge.

Trump could also make this provision a part of his infrastructutre-spending initiative (or boondoggle, if you prefer), and deny this additional money to states which do not comply with federal immigration laws. But there again, the RINOs will likely support Democratic initiatives to strip this provision out of the spending bill.

Correction: A commenter says it's the anti-commandeering doctrine, as it's not an actual clause in the Constitution, but an implication from it, announced in case law. A look-up online finds a lot of references to the anti-commandeering clause., but I think the commenter is right -- even if that terminology is widespread, it's misleading and erroneous.

tsrblk says the Medicaid funds cut-off from Obamacare was unconstitutional because it said the government would cut off money already authorized by law to flow to the states, and not just "new money" authorized in the bill itself.

That sounds right to me -- however, that means the only way Trump could strip money from cities is to propose new money funding of them, then withhold it if they don't comply.

That strikes me as two steps back and then only one step forward. I don't want to have to authorize new federal spending just to be able to threaten to take it away.